The present invention relates to a radio communication system which uses multiple bands or a plurality of different radio frequencies, and in particular relates to a radio communication system, and a transmission apparatus and reception apparatus, which use different radio transmission methods (radio transmission parameters) for each band or radio frequency.
Second-generation mobile telephone systems use a plurality of frequency bands, such as the 800 MHz band and the 1.5 GHz band. And, in the IMT-2000 third-generation mobile telephone system, currently the 2 GHz band is being used, but usage of the 800 MHz band in the near future is also being studied. Thus the use of a plurality of frequency bands in a single mobile telephone system is a well-known fact.
In a multiple-band radio communication system, that is, a radio communication system using a plurality of bandwidths (bands), or in a multi-carrier radio communication system using a plurality of different radio frequencies, in the prior art the same radio parameters which are synonyms of the radio formats were used for all. That is, the radio format comprises, for example, (1) the length of interpolated pilots necessary for channel estimation, (2) the length of the guard interval GI to prevent intersymbol interference, and (3) the number of subcarriers in multi-carrier transmission and the interval between subcarriers; in the prior art, these radio parameters (formats) were the same regardless of the radio frequency or band. However, if the frequency band used is different, then propagation characteristics are different, and so reception performance is accordingly different. FIG. 15 explains multi-band transmission; in order to simplify the explanation, the frequency bands are limited to the 1 GHz band and the 2 GHz band, but there is no need to limit the bands to these frequency bands, nor is there a need to limit transmission to two bands.
(1) Radio Transmission System with Pilot Interpolation
In a radio transmission system in which the 1 GHz and 2 GHz frequency bands are used, even when the motion velocity is the same, one fading velocity is twice the other due to the frequency bands used. Hence if the pilots of the same length are interpolated for channel estimation, the channel estimation precision will be different for 1 GHz and for 2 GHz, and reception performance will be poorer in the 2 GHz band compared with the 1 GHz band. However, in the prior art, as shown in FIG. 16, the lengths of the pilots PL1, PL2 interpolated into the data DT1, DT2 regardless of frequency band are the same, and so there has been the problem that the channel estimation precision is worsened for the 2 GHz band.
FIG. 17 shows the configuration of such a transmission apparatus of the prior art, in which the pilot length is held constant regardless of the frequency band; FIG. 18 shows the configuration of a reception apparatus.
In the transmission apparatus, the modulation portion 1a performs for example QPSK modulation of the transmission data, the pilot insertion portion 1b inserts pilot signals PL into the QPSK in-phase component and quadrature component, the 1 GHz transmitter 1c up-converts the frequency of signals with pilots PL to 1 GHz and transmits the signals, and the 2 GHz transmitter 1d up-converts the frequency of the signals with pilots PL inserted to 2 GHz and transmits the signals. Pilot insertion may be performed before QPSK modulation.
In the reception apparatus, the 1 GHz receiver 2a down-converts received 1 GHz high-frequency signals to baseband signals and inputs the signals to the selection portion 2c, and the 2 GHz receiver 2b down-converts received 2 GHz high-frequency signals to baseband signals and inputs the signals to the selection portion 2c. The selection portion 2c selects baseband signals output from the receiver indicated by a 1 GHz/2 GHz selection signal SEL output from a control portion, not shown, and inputs these signals to the pilot extraction portion 2d and demodulation portion 2e. The pilot extraction portion 2d extracts pilots from the input signals, and the channel estimation portion 2f uses the extracted pilot signals and known pilot signals to estimate the channel (path propagation characteristics). The demodulation portion 2e performs channel compensation of data signals based on the channel estimation value, and then demodulates the transmission data.
In this way, the transmission apparatus inserts pilot signals of the same length for both 1 GHz/2 GHz, and uses the same radio format to transmit the radio signals. Hence when demodulating data transmitted at 2 GHz, the channel estimation precision is worsened, and so high-precision data demodulation is not possible.
(2) Radio Transmission System with Guard Intervals Inserted
In a radio transmission system in which guard intervals GI are inserted in order to prevent intersymbol interference, the necessary guard interval length differs depending on the positional relation between the base station and the mobile station. For example, propagation losses differ at 1 GHz and at 2 GHz, and it is known that signals reach farther at 1 GHz, and the delay spread is longer in the 1 GHz band. The guard interval length is generally determined according to the longest delay spread. That is, when the guard interval length is the same (same radio format) for each band, the positional relation between base station and mobile station for which the delay spread is longest is assumed to determine the required guard interval length. FIG. 19 is an example of a radio format of the prior art; the length of 1 GHz/2 GHz guard intervals GI is determined based on 1 GHz delay spreading. From the above, in the 2 GHz band the guard intervals are too long, that is, there is the problem that transmission efficiency is worsened by providing excess guard interval length.
FIG. 20 is an example of a transmission apparatus in a radio transmission system in which guard intervals GIs are the same; FIG. 21 shows the configuration of a reception apparatus, in an example of multicarrier transmission by Orthogonal Frequency Division Multiplexing (OFDM), in which data is transmitted from a transmitter with guard intervals of the same length inserted at 1 GHz/2 GHz.
In the transmission apparatus, the serial/parallel conversion portion 3a1 of the multicarrier modulation portion 3a converts the transmission data into N parallel data symbols, the IFFT portion 3a2 performs IFFT processing of the parallel data symbols into N subcarrier components, and the parallel/serial conversion portion 3a3 converts the N-symbol IFFT processing result into serial data, which is output. The guard interval addition portion 3b adds a guard interval of constant length, set in advance, to the beginning of N-symbols, the 1 GHz transmitter 3c up-converts the frequency of the signals with guard intervals inserted to 1 GHz and transmits the signals, and the 2 GHz transmitter 3d up-converts the frequency of the signals with guard intervals inserted to 2 GHz and transmits the signals.
In the receiver, the 1 GHs receiver 4a down-converts the 1 GHz high-frequency received signals to baseband signals and inputs the signals to the selection portion 4c, and the 2 GHz receiver 4b similarly down-converts 2 GHz high-frequency received signals to baseband signals and inputs the signals to the selection portion 4c. The selection portion 4c selects baseband signals output from the receiver indicated by a 1 GHz/2 GHz selection signal SEL output from a control portion, not shown, and inputs these signals to the guard interval removal portion 4d. The guard interval removal portion 4d removes guard intervals from the input signals, and inputs the result to the FFT portion 4e. The FFT portion 4e parallel-converts the input signals into N-symbols, then performs N-point FFT processing, serial-converts the FFT result, and inputs this to the demodulation portion 4f. The demodulation portion 4f demodulates the transmission data from the input signals.
In this way, the transmitter inserts guard intervals of the same length, and uses the same radio format for transmission of radio signals at both 1 GHz/2 GHz. As a result, the guard intervals are too long at 2 GHz, and transmission efficiency is worsened.
(3) Multicarrier Transmission System Using Multiple Bands
As shown in FIG. 22, in radio communication systems which perform multicarrier transmission by the OFDM method in each of multiple bands (the 1 GHz band and 2 GHz band), when frequency fluctuations due to fading occur, the orthogonality between adjacent subcarriers is degraded. The degree of degradation of orthogonality differs with the frequency band used. That is, even when the motion velocity is the same, in the 2 GHz band the amount of frequency fluctuation is twice that in the 1 GHz band, and so the amount of degradation is greater than in the 1 GHz band.
In OFDM, transmission signals are serial/parallel-converted (converted into N parallel signals), the signal rate is lowered, and the N transmission signals are each allocated to a subcarrier and transmitted. The subcarrier interval or band width is determined by the signal rate (=1/T Hz) after serial/parallel conversion. Subcarrier intervals are set to 1/2T intervals so that subcarriers are orthogonal on the frequency axis. In this OFDM transmission method, as explained above, there is frequency fluctuation due to multipath fading, and when the orthogonality between subcarriers is degraded, performance deteriorates. Hence there is a need to set the frequency intervals in advance taking this fluctuation into account, so that degradation in a band does not occur. However, In multiple-band radio transmission systems of the prior art, subcarrier intervals in a band are the same at 1 GHz and at 2 GHz. A radio communication system performing multicarrier transmission using OFDM in multiple bands (the 1 GHz band and 2 GHz band) has the same configuration as in FIG. 20 and FIG. 21.
There exists technology in which, when the degree of signal degradation differs with the frequency, signals for a frequency channel with a satisfactory reception state are selectively received (JP 2002-64458A). In this technology of the prior art, in a multi-frequency network in which a plurality of transmitting stations transmit the same content at different frequencies, a receiving station detects the reception levels of signals sent over two channels with different frequencies, and uses the signals in the channel with the higher reception level for content restoration.
Also, there exists technology in which a frequency, time, or direction for which radio communication interference is anticipated is determined, and radio communication is performed avoiding these conditions (JP 2002-300172A).
However, these technologies of the prior art do not improve the reception performance in each band or at each frequency in a multiple-band radio communication system or in a multicarrier radio communication system.